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Community and Q&A

Retrofitting insulation: heat loss/gain where ceiling meets the wall

Bdgray | Posted in Green Building Techniques on

Hello all,

As part of my efforts to air seal and insulate my home, I just purchased a new FLIR infrared camera. First, what a cool tool for an energy nerd! It has instantly identified areas of the home on which to focus.

For the most part, everything I found was straightforward – a couple of electrical holes that need to be sealed from the attic and a painful realization that can lights, even with rockwool covers, are big areas of loss. There is however one area that I don’t know how or even if I can address. On virtually every exterior wall there is a heat plume coming from where the ceiling and exterior walls meet. I’m guessing this is normal since it is really hard to insulate at these corners. But this is going to bug me until I know whether or not I can address it.

I have three thoughts on this on how to address this. I am looking for feedback:

1) I own 4″ thick 4×8 sheets of leftover polyiso insulation. With a little effort I could cut these into triangular sections that would fit behind crown moulding (which does not currently exist). That would give me a pretty thick triangular section of polyiso blanketing the corners.
2) Something else that I’ve not thought of.
3) Give up and accept that my house is good enough.

Our home is 60 years old and will never win any world records for energy efficiency. However, we’ve made a ton of progress. We have cut our energy bill in half from lots of retro-insulating, air sealing, and a new 95% radiant system. More importantly the house is more comfortable. Anyway, I’m not trying to make this home LEED certified. Just curious if there is a relatively easy way to reduce the heat loss in these corners.


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  1. Expert Member
    Dana Dorsett | | #1

    Are you talking ceilings, or are you talking roofs?

    The band joists on first floor ceilings are often uninsulated, but can often be retrofitted with blown fiber. A typical solution for insulating band joists at the end of a ceiling/floor joist bay is to drill a 2-5" hole in the ceiling with a hole-saw, insert a feed-bag, and dense-pack it with cellulose.

    At top floor ceilings next to the eaves the venting details sometimes get in the way of just filling the joist bays with insulation out over the top plate of the walls, but there are also often electrical penetrations in the top plates to be sealed that contribute to the problem, that should be sealed first.

  2. GBA Editor
    Martin Holladay | | #2

    Your suggestion comes up about once a year. Every time the suggestion comes up, I provide a similar answer.

    I'll give you two links to previous Q&A threads on the topic. My answer in the first link is a little more complete than my answer in the second link:
    Weird top-plate area idea
    Interior insulated cornice for use under low-heel roof trusses?

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